Distribution of a Monovalent Anion in Various Ionic Liquid/Water Biphasic Systems: Relationship of the Distribution Ratio of Picrate Ions with the Aqueous Solubility of Ionic Liquids

The distribution of picrate anions in various ionic liquid (IL)/water biphasic systems was investigated at 298.2 K. The ILs were 1-butyl-3-methylimidazolium hexafluorophosphate, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, 1-hexyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide, 1-methyl-3-octylimidazolium tetrafluoroborate, 1-methyl-3-octylimidazolium hexafluorophosphate, 1-methyl-3-octylimidazolium bis(trifluoromethanesulfonyl)amide, 1-methyl-3-octylimidazolium bis(pentafluoroethanesulfonyl)amide, 1-butyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)amide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethanesulfonyl)amide, methyltrioctylammonium bis(trifluoromethanesulfonyl)amide, 1-butylimidazolium bis(trifluoromethanesulfonyl)amide, and 1-butylpyrrolidinium bis(trifluoromethanesulfonyl)amide. The distribution ratios in dilute conditions ( D °) and the aqueous solubilities of the ILs (square root of the solubility product, K sp 1/2 ) were determined. The extractability of the picrate anion generally increases with increasing hydrophobicity of the IL cation (C + ) and increasing hydrophilicity of the IL anion (A – ). For the ILs with different C + but the same A –, the log D ° vs log K sp 1/2 plot generally gives a linear relationship with a slope of −1; when the ILs have similar K sp 1/2 values, the D ° value decreases in the C + order, protic cations ≫1,3-dialkylimidazolium cations > other cations. For the ILs comprising different A – but the same C +, the log D ° versus log K sp 1/2 plot is close to a linear line with a slope of 1. These regularities can be explained on the basis of the extraction mechanism including both the ion pair extraction with C + and the ion exchange with A – .